# NCERT Class 7 Chapter 4 Heat YouTube Lecture Handouts

Watch Video Lecture on YouTube: NCERT Class 7 Science Chapter 4: Heat

NCERT Class 7 Science Chapter 4: Heat

## What Will We Learn?

• Conductors and Insulators

• Heat and Temperature

• Measuring Temperature

• Units

• Thermometers

## Questions?

• Why do we use copper bottom on SS utensils?

• Can we use lab thermometer to measure body temperature?

• When you heat the air filled balloon it bursts why?

## It All Started with Burning of Hands?

• Note that hot and cold are relative- Objects colder than us seem cold and objects warmer than us appear hot.

• Keep hand on ice and then even cold water would appear warm.

• Warm your hands on a heater and then same water would appear cold.

• So how do we communicate- standardization.

• Man touching a hot frying pan and running around (AE)

## Heat is a Form of Energy

• Heat is a form of energy it has both macroscopic and microscopic implications.

• At macroscopic level things with heat energy feel warm.

• At microscopic level things with heat energy have more active particles.

• In solids more vibrations

• In liquids and gases molecules move faster.

## Heat Increases Movement of Particles (Vibration in Solids)

• Heat is a form of energy it has both macroscopic and microscopic implications.

• At macroscopic level things with heat energy feel warm and they expand. If you heat a air filled balloon it will burst.

• At microscopic level things with heat energy have more active particles.

• In solids more vibrations

• In liquids and gases molecules move faster.

• Similar to children eating lot of candy and jumping around

• In presence of teacher (in their seats)

• In absence of teacher (out of their seats jumping around)

• More candy more movement- they require more space- expansion.

## Effects of Heat- 3 Main Effects

1. Expansion (with Few Exceptions)

2. Phase Change (Solid->Liquid, Liquid->Gas)

3. Hotness/Coldness or Temperature

• Heat is a form of energy it has both macroscopic and microscopic implications.

• At macroscopic level things with heat energy feel warm and they expand. If you heat a air filled balloon it will burst.

• At microscopic level things with heat energy have more active particles.

• In solids more vibrations

• In liquids and gases molecules move faster.

• Similar to children eating lot of candy and jumping around

• In presence of teacher (in their seats)

• In absence of teacher (out of their seats jumping around)

• More candy more movement- they require more space- expansion.

## Transfer of Heat (3 Methods)

• Noticed your mom the kind of sound than comes when she pours cold water over hot pan- the pan quickly cools down.

• Hot coffee cup transfers heat to air and becomes colder

• Cold ice cube takes energy from surroundings, melts and becomes warmer

• The greater the difference between and hotness or coldness and surroundings the faster the change.

• Ice cube in summer vs. winter or coffee cup in summer and winter.

• If temperature becomes same no transfer.

• One body becomes hot other becomes cool. In or example the air around ice looses heat and becomes cooler. Similarly air around cup takes energy and becomes warm.

## Conduction

• Heat one end of rod. Particles get energy start vibrating- similar to students sitting at their place- they cant just move- are stuck at one place.

• Vibration travels to nearby particles and so on ultimately other end becomes hot.

• Thus some material are good conductors and others are bad- more on that later.

• Students passing book

## Convection

• Heat a pot of water

• Molecules of water near the flame are excited and start moving around- unlike solids they themselves can move. Other molecules come in their place and they also get heated etc.

• During movement the energetic molecules hit other molecules and transfer some of the “heat” energy

• Now in presence of gravity the hotter water becomes lighter and so moves up. Thus water above comes down and gets heated. This creates a circulating pattern called convection.

• In absence of pot the air itself gets heated by convection. As hot air moves up, convection is mainly present above the flame. So above the flame is the hottest.

• Students themselves move

• Radiation: Generated and absorbed mainly by photons. Radiation is particle transport from one place to another that is not part of the bulk medium.

• Students just throw the book

• Conduction: Molecules exciting their neighbors successively.

• Convection: Molecules heated like in conduction, but then move to another location.

• Black absorbs most of the light and hence becomes the hottest white is the coolest. Light colors are good in summer

• Blue object absorbs other colors except blue.

• That’s why solar cookers/solar cells are black.

## Modes or Transfer: Summary

Consider the fire on the stove and a pot full of water. The pot is getting hot by conduction, the water by convection. Air above the stove is hot due to convection however person on the side feels the heat due to radiation.

1. Metal

2. Diamond

## Insulators

• The handle of pan can be held even when the metal pan is very hot or cold. Why?

• Its not that insulators will not become hot – they will become hot or cold more slowly.

## One Blanket or 2?

• Why do we feel cold at all?

• So we need to wear insulator in cold?

• Which clothes to wear in winter? Woollen or cotton?

• Why are woolen clothes so effective- trapped air

• Why not wear the thick jackets even in summer we wont feel hot right?

• In winter- which is better one thick blanket or 2 thin blankets?

## Do We Need to Measure Hotness/Coldness?

• How to communicate weather? (add diagram of summer or cold day)

• How do we know if we have fever? (add picture of sick child with thermometer)

• How do we even cook? (add picture of water boiling)

• Of course science experiments! (add picture of chemistry with burning)

## Thermometers: Clinical and Lab

• Why don’t we measure temperature by hand?

• One of the early scientists to start developing a way of measuring temperature was Galileo Galilei.

• These devices were called “thermoscopes” because they did not actually have a scale which measured temperature.

• Mercury thermometers: Object expands on heating and contract on cooling .

• Expansion chamber- At the top- only in clinical

• Bulb- At bottom- made of metal

• Capillary and Stem- Special shape of stem

• Contraction Chamber/Bend- Slows the movement down

• Why choose mercury- 2 reason good conductor and more expansion.

## Temperature

1. The degree of hotness or coldness of a body or environment is called temperature.

2. Expressed by a number on a scale.

• Measures average energy of moving molecules/atoms- Increases with energy.

• The degree of hotness or coldness of a body or environment is called temperature.

• Heat flows from higher temperature to lower temperature until it becomes equal.

• Temperature is a measure of the ability of a substance to transfer heat energy to another physical system. The higher the temperature difference, the greater the tendency to transfer heat.

## Units of Measuring Temperature (Temperature Scales)

• German engineer named Fahrenheit, who was a thermometer manufacturer proposed Fahrenheit scale with 180 degrees between melting ice and boiling water.

• A few years later, a Swedish astronomer named Celsius proposed a different scale of 100 degrees- of course it was reversed.

• Kelvin scale absolute zero means – no vibrations in particles- no degree in kelvin. Bose Einstein condensate (Satyendra Nath Bose)

• Understand the scale- look at close-up 1 small division is 0.1 degree

• Note that nine lines form 10 divisions.

## Important Temperatures

1. Freezing point of water

2. Body temperature- around

3. Clinical thermometer range- to

4. Boiling point

5. Lab thermometer range - to

• Coldest parts of space appear to be about 3 Kelvin

• The closest we have gotten to absolute zero is few billionths of a degree above zero

• The boiling water has temperature of 100ºC, it can’t be measured using clinical thermometers.

• Don’t wash a clinical thermometer in boiling water as It is designed to measure a small range of temperature around 37 degree Celsius.

## Right Way to Measure (Clinical)

• Wash/antiseptic.

• Shake until mercury column falls.

• Keep in touch with body part (for 1- 2 min).

• Carefully read the temperature, notice what small scales measure.

• Do not break.

• Mercury is toxic.

## Right Way to Measure (Lab)

• Understand the scale

• Thermometer should be upright

• Do not touch the sides or the bottom only water.

• Bulb should be completely dipped in water.

• Dip until mercury level becomes stable.

• If no kink or contraction then read while in water.

• of course clinical thermometer always have the kin

## Lab vs Clinical Thermometer

• Why cant we use our hands to measure temperature?

• One hand ice and other hand hot water- now place both hands in same water. The hand which was in ice feels that water is warm while other hand feels that water is cold.

• So hotness and coldness experienced by skin is relative etc.

## Modern Digital Thermometers

• Make diagram transparent

• Mercury thermometers: Object expands on heating and contract on cooling . We don’t wash a clinical thermometre in boiling water as It is designed to measure a small range of temperature around 37 degree Celsius.

• The boiling water has temperature of 100ºC- cant be measured using mercury thermometers

• Why do we use copper bottom on SS utensils?

• Can we use lab thermometer to measure body temperature?

• When you heat the air filled balloon it bursts why?

• The pot and the stove are obviously in contact with each other. Therefore conduction plays a role here. If you have an old pot, with a warped bottom, it will heat up slower, because the contact surface between pot and stove is smaller.

• When you hold your hand over the stove (not touching it), you can feel the heat. The air above the stove is heated and because it is a gas, moves upward. This is convection. The bottom of the pot and the surface of the stove are not 100% flat. That’s why there will be little pockets of air underneath the pot, even if you place it on the stove.

• If you heat up the stove as much as you can, it will glow red. This is a visible sign of radiation. I’d assume that even if not visibly glowing, the stove radiates heat, too. In those areas where the stove and the bottom of the pot are not in contact, radiation transports heat from the stove to the heat.

Get unlimited access to the best preparation resource for UGC - Get detailed illustrated notes covering entire syllabus: point-by-point for high retention.

Developed by: